No. 381. A HALF-CENTURY OF EVOLUTION. 6 
43 
coda, Phyllopoda, the sole Cambrian form (Protocaris marshi) 
related to the modern Apus, and the Phyllocarida. Of these 
the barnacles and ostracodes with their multivalve or bivalve 
carapaces are the most specialized, and in the case of the 
former the process of modification due to this fixed mode of 
life must have required ages, as must also the development 
of that highly modified vermian type, the Brachiopoda. 
Indeed, the three lines of descent which resulted in the arthro- 
podan phylum, as it now exists, unless there were three inde- 
pendent phyla, were perhaps initiated before the Cambrian. 
These lines are: (1) the Trilobita, with their probable succes- 
sors the merostomes and arachnids ; (2) the Crustacea ; and (3) 
the myriopods and insects. Of the third line Peripatus or a 
Peripatus-like form was the earliest ancestor, which of course 
must have been terrestrial in habits, though its forefather may 
have been some fresh-water leech-like worm. We venture to 
state that it is not wholly impossible that so composite a type 
as Peripatus, which bears at least some of the marks of being 
a persistent type, took its rise on the continental land of the 
Precambrian. , 
In the Precambrian time was also solved the problem by the 
mollusks of producing a spiral univalve shell ; for while a large 
proportion of the Gastropoda were protected by patella-like 
shells of simple primitive conical form, with these coexisted, in 
the Lowest Cambrian, forms with spiral shells, such as Platyceras 
and Pleurotomaria. The comparative abundance of those highly 
modified mollusks, the Pteropoda, in the lowest or Olenellus 
Cambrian strata, strongly suggests that their divergence from 
the more generalized gastropod stem, and their adaptation to a 
surface or pelagic life, must have taken place long anterior to 
the dawn of the Cambrian.) With them must have lived 
a variety of other surface forms besides Rhizopoda, whose 
young served as their food. The members of all classes of 
the Cambrian were carnivorous, feeding on the protoplasm of 
1 Dr. Matthew has discovered at St. John, N. B., a still lower and older bed, 
containing no Olenellus, but Foraminifera (Orbulina and Globigerina), sponges, 
Pteropoda, Pelagiella, which was probably an oceanic heteropod, very primitive 
brachiopods, with Ostracoda and six genera of trilobites. 
